Physiological changes occur in the cat spinal dorsal horn after partial deafferentation. Two common explanations are: (a) remaining afferents sprout, forming new synapses with the partially denervated postsynaptic population; and (b) the synaptic strengths of remaining synapses are in- creased. This study will test two closely related hypotheses: (a) The bouton distribution of remaining afferents changes; and (b) the average field potential contribution per bouton increases in magnitude. The latter may be an indication of a change in average synaptic strength. The bouton distributions and a measure of average field potential contribution per bouton of single large diameter cutaneous afferents will be compared on both sides of animals which have been partially deafferented, chronically on one side and acutely on the other. Deafferentations will consist of spared root and severed cutaneous nerve preparations. Afferent axons will be penetrated with an HRP-filled microelectrode, and the axons' cutaneous receptive fields and receptor types will be determined. Then the longitudinal distributions of the cord dorsum potentials (CDPs) evoked by single spikes from the afferents will be measured. The axons will then be loaded with HRP. The longitudinal distribution of boutons will be determined from the histochemically processed cord tissue, and the average contribution per bouton to the CDPs will be estimated. Asymmetries in these measures, between chronic and acute sides, may indicate sprouting or strengthening of synapses. The results of this research will go a long way toward elucidating the relative contributions of sprouting and strengthening of synapses to changes occurring in the spinal cord after partial deafferentation. Mechanisms underlying such changes may be crucial to the development of therapies for spinal cord or peripheral nerve injury. These techniques will be used in future studies to assess the effects of a variety of spinal lesions and potential therapies for treatment. This could prove instrumental in developing methods for treatment of spinal cord injuries in humans.